1. Field of the Invention
The present invention relates to a battery, in particular a traction battery.
2. Description of the Prior Art
It has become apparent that in the future, both in stationary applications (e.g. wind power stations) and in vehicles (e.g. in hybrid and electric vehicles), there will be increased use of new battery systems that will be subjected to very strict requirements with regard to reliability. The reason for these strict requirements is that a failure of the battery can result in a failure of the overall system (e.g. in an electric vehicle, a failure of the traction battery results in a so-called “stranded vehicle”) or even in a safety-related problem (in wind power stations, for example, batteries are used for adjusting the rotor blades so as to protect the system from impermissible operating states).
The schematic wiring diagram of a battery system according to the current prior art is shown in FIG. 9. To achieve the required performance and energy data with the battery system, individual battery cells are connected in series and partially also in parallel. In addition to the battery cells, the battery system also has a so-called charging and disconnecting device that is shown in FIG. 9 as being situated between the plus pole of the battery and the battery cells, without limiting general design freedom. The circuit breaker TS can be used to switch the battery on and off in monopolar fashion. As an optional functional unit, FIG. 9 shows another disconnecting device with which the battery—if necessary by means of another circuit breaker—can be switched off in bipolar fashion. In the charging and disconnecting device, there is also a so-called charging switch with which a charging resistance can be connected between the battery cells and the externally connected systems in order to limit the equalizing currents when switching on the battery. In such a switching-on procedure, if the circuit breaker is open, then the charging switch in the charging and disconnecting device is first closed and also—if provided—the circuit breaker in the optional disconnecting device on the minus pole of the battery system is closed. Then the input capacitances of the externally connected systems are charged via the charging resistance. If the voltage between the plus pole and minus pole of the battery system deviates only insignificantly from the sum voltage of the battery cells, then the charging procedure is terminated by the closing of the disconnecting switch in the charging and disconnecting device. The battery system is then connected to the external systems in a low-impedance fashion and can be operated with its specified performance data. With the above-explained procedure, the equalizing currents that occur between the external systems and the battery system at the switching-on of the battery system can be limited to permissible values.
The reliability of the battery system is indicated by the failure rate. The failure rate describes the number of failures to be expected on average in a given period of time.
The failure rate of a battery with a series circuit of individual cells can be determined as follows:Failure ratetraction battery=1−(1−failure ratecell)number of cells  (1)
The traction battery of an electric vehicle with a series circuit of 100 cells and a failure rate of 100 ppm/cell in the given period of time, for example, thus yields the following:
                                                                        Failure                ⁢                                                                  ⁢                                  rate                                      traction                    ⁢                                                                                  ⁢                    battery                                                              =                              1                -                                                      (                                          1                      -                                              100                        ⁢                                                                                                  ⁢                        ppm                                                              )                                    100                                                                                                        =                              9.95                ⁢                                  %                  0                                                                                        (        2        )            
With very low failure rates of the individual battery cells (e.g. failure ratecell<10/00 in the given time period), the failure rate can be calculated approximately as follows (the power series expansion of the binomial series interrupted after the first term):Failure ratetraction battery≈number of cells*failure ratecell  (3)
The failure rate of the traction battery in question is therefore about 100 times greater than the failure rate of an individual cell. The failure rate of the individual cells must therefore be smaller by a factor of approximately 100, given the required values for the failure rate of the battery system. For a battery system with 100 series connected cells, if a failure rate of 100 ppm in the given period of time is required, then the cells must have a failure rate of 1 ppm during this period of time. This is a requirement that is extremely hard to fulfill.